This invention relates to a process and an apparatus for welding straps made of thermoplastic plastics, particularly packaging straps made of polypropylene or the like, which are heated over a limited welding region on their mutually facing faces and are pressed on to each other under the action of pressure.
During the welding of straps made of plastics it is customary to supply or to generate welding heat for a predetermined period of time which experience has shown is sufficient to soften the straps sufficiently in the weld region and intimately to fuse them to each other on the application of pressure. It has been shown in this respect, however, that the strength of the weld is not always satisfactory, and that welding has not been performed correctly. The reason for this is based on environmental effects and on irregularities in the supply of heat, which occur, for example, if there is a drop in the voltage of an electrical source of heat and thus in the temperature also. Particularly when employing friction welding devices which are operated by an accumulator independently of the mains, the number of revolutions also becomes less as the voltage decreases, so that the oscillating device, which moves the superimposed straps towards each other in order to generate frictional heat, moves more slowly. The resulting welding time is then no longer sufficient to generate the requisite welding heat, and intimate fusion of the straps in the weld region no longer occurs.
The object of the invention is to avoid these disadvantages and to provide a process and an apparatus with which plastics straps can be welded to each other in a simple and reliable manner, even under changing environmental effects. This object is achieved by the invention in that in order to ensure correct welding the change in the total thickness of the straps which are welded to each other is determined during the welding operation and is compared with a predetermined set value.
The process according to the invention has the advantage that the strength of each weld is tested, since in fact the total thickness of the straps which are welded to each other at the weld location depends on the softening of the plastics and the intimate fusion thereof, and is thus at the same time a measure of the strength of the welded joint. Depending on a predetermined total thickness, the supply of heat to the straps which are welded to each other is not interrupted until satisfactory fusion of the straps in the welding region is achieved.
Since the duration of the supply of welding heat no longer plays a part, perfect welding operations can be carried out using accumulator-operated devices, even if the voltage of the accumulator has already dropped, since the lower temperature is compensated for by a longer time of action.
An apparatus for welding straps made of thermoplastic plastics, particularly of packaging straps made of polypropylene or the like, which is provided with a heating device for heating mutually facing faces of the straps which are to be joined to each other, and which is provided with a pressure element which can be moved at the weld approximately perpendicularly to the weld faces in order to compress the straps which are superimposed on a supporting surface, is characterised according to the invention in that the heating device comprises a switch-off device, which is operated directly or indirectly by the pressure element as soon as the thickness of the weld, and thus the distance between the pressure element and the supporting surface, falls below a predetermined set value.
A welding apparatus which is constructed in this manner does not automatically switch off the supply of welding heat until sufficient heating and fusion of the straps is achieved at the weld, which is manifested by a decrease in the total thickness of the superimposed straps at the weld.
In the apparatus according to the invention, a signal transmitter can be associated with the pressure element, which signal transmitter determines the changes in the distance between the pressure element and the supporting surface and feeds corresponding signals to an evaluation electronics unit, which generates a switch-off signal in order to switch off the heating device when the distance between the pressure element and the supporting surface reaches a predetermined set value. Sensors can be used here which determine the changes in distance in a contactless manner, e.g. by an optical route, electromagnetically, or by ultrasound.
In a mechanically operated welding apparatus, the pressure element can be connected to a sensing element which senses the surface of the superimposed unwelded straps and which comprises an operating element for switching off the heating device. The pressure element here is displaceable to a limited extent in relation to the sensing element in a direction towards the supporting surface, and during its displacement acts on the operating element, which switches off the heating device as soon as the weld has reached the predetermined thickness.
The operating element can be a lever which is swivel-mounted on the sensing element, which lever is acted upon by a driver disposed on the pressure element and which comprises an adjustable switching finger at one free end, which switching finger operates a first switch when the displacement of the pressure element in relation to the sensing element reaches a predetermined value.
By adjusting the switching finger, the set value for the total thickness at the weld of the straps to be welded to each other can be adjusted.
It is advantageous if the first switch comprises a normally closed contact and is connected in series with a second switch which switches on the heating device if said second switch is operated during the action of pressure of the pressure element.
A particularly simple form of construction is achieved if the sensing element comprises a driver pin, the free end of which fits into an elongated recess, which extends in an axial direction, in the pressure element, in which recess it is held pressed by a spring element against the end face of the recess which faces the supporting surface, and in which recess it can move against the action of the spring element when the pressure element moves in relation to the sensing element in a direction towards the supporting surface during welding.
The welding heat can be generated at the weld location in various ways. In one preferred embodiment of the invention, the heating device can be a friction welding device, the oscillating lever of which moves a pressure piece which detects the upper strap, and which is part of the pressure element and is pressed together with the latter in a direction towards the supporting surface and is moved transversely to the plane of the strap by the oscillating lever and generates frictional heat between the straps at the weld location.
In another embodiment, the heating device can comprise a heating resistance wire which is disposed at the weld location between the straps which are to be welded to each other, which heats the straps at the weld and which is embedded in the straps when it is compressed by the pressure element and remains there.
The process according to the invention, and preferred embodiments of the apparatus according to the invention, are explained in more detail with reference to examples of embodiments by the description given below and by the drawings, where: